This invention relates to a novel porous fabric and a process for manufacturing same, and more particularly to a novel porous fabric having surface openings of at least 100,000 perforations per square inch and a process for forming such novel porous fabric from a composite fiber-film substrate.
Many non-woven and (continuous filament spun-bonded fabric or webs have found use in the market as substitutes for textile materials. Such webs consist of randomly laid fibers, either in the form of short staple lengths or continuous filaments, which are undrawn or only partially drawn, and have therefore not obtained their optimum strength. In my copending application U.S. application Ser. No. 900,720 filed Apr. 27, 1978, there is disclosed in a preferred embodiment, a process and apparatus for bi-axially and incrementally stretching a web of synthetic fiber in a first and second station wherein the first and second stations are provided with sets of rolls having grooves parallel and perpendicular, respectively, to the axis of each set of rolls. The groove pattern of each set of rolls is such that the distance between grooves is less than 1.0 millimeters times the web basis weight in grams per square meter. The non-woven web of synthetic material is stretched in a manner to affect uniform stretching between bonding points of each individual fiber thereby producing a web of bi-axially larger dimension and correspondingly reduced base weight.
In U.S. Pat. No. 4,116,892, assigned to the same assignor as the present invention, there is disclosed a process for the selective stretching of incremental portions of a substrate of a synthetic thermoplastic material selected from the group consisting of a thermoplastic orientable polymer or a blend of a thermoplastic orientable copolymer in which there is admixed an incompatible second phase selected from the group consisting of an incompatible polymer or inorganic material. The substrate is stretched in grooved roller pairs by controlling the velocity of introduction of the substrate to maintain the velocity substantially identical to the surface velocity of the roller pair to form an opaque, low density porous sheet useful as a printing substrate, such as synthetic paper; as a substitute for leather; as a highly fibrillated sheet which can easily be shredded into fine fibrils to be used as substitutes for paper-making pulps, or as a filter material, such as battery separators.
In U.S. Pat. No. 2,994,617 there is disclosed a process for producing multi-layer, water-vapor permeable, scuff resistant sheet materials comprising a compressed interconnecting pore-containing layer of non-woven matted fibers bound together with an extensible polymeric binder and adhered thereto a surface layer of a perforated, extensible, scuff-resistant polymeric material. Porosity of the pore-containing layer is effected by stretching a composite layer generally after perforation of the surface layer of the polymeric material. Formation of the base layer requires permeation of the non-woven mat of fibers with a solution or suspension of binder having flow properties at a temperature at least 50.degree. F. below the deformation temperature of the fibers of the non-woven mat. The resulting sheet of material is subjected to the steps of hot pressing and pressure cooling to solidify the binder and form the desired stratum. Surface coating, performation of the surface coating and stretching of the stratum follow to form the resulting product having a surface with from 300 to 30,000 perforations per square inch and a porosity measured by the leather permeability value (LPV).